This invention relates to an apparatus for detecting a fault of controls and a plant which is an object for control in an automatic control device, or more in particular to a fault-detecting apparatus for the automatic control device, comprising a model simulating the characteristics of the controls and the plant for detecting a fault of the plant or like by a model reference method in which the output of the model is compared with that of the plant.
Automatic control devices are widely used industrially, and many of them are what are called closed-loop control devices using feedback signals. In other words, a plant which is an object for control is operated in response to an output signal from the controls, which in turn are operated in response to a deviation between the plant output and a target signal associated therewith. In such a system, the plant output is generally called a feedback signal. In the event of a fault of this control device, the plant will unexpectedly experience a run away condition or shut down without being controlled as directed by the target signal. Generally, the shut-down of a plant is a safe reaction to occurrence of a fault and does not pose a great problem. In the case where the object plant is connected to another plant, however, such another plant is undesirably affected by the fault. This effect is larger, the larger the size of the system or the more complicated it is. If the fault is a run-away, the effect is much greater, sometimes leading to the breakdown of devices and equipment.
For these reasons, various methods have so far been suggested for detecting a fault of the controls at an early time in order to take action suitable to the plant involved. One of the conventional methods for detecting a fault of the controls is by monitoring a deviation between a target signal and a feedback signal, such as disclosed by the Japanese Patent Application Publication No. 6815/72, which is based on the theory that "under the normal condition, the target signal substantially coincides with the feedback signal". Thus, it is possible to detect a fault with high accuracy under normal conditions, but many problems are posed under transient conditions. When the target value is changed, for instance, the absolute value of the deviation signal is increased. From this mere fact, it is impossible to judge whether it is due to the change in target value (normal) or a fault of the devices (abnormal). Further, in the case where response of the plant is delayed, the condition of a large deviation continues for a long period of time. If the detection sensitivity of the fault-detecting apparatus is increased, an erroneous judgement is likely to be made, while in order to reduce the chance of erroneous judgement, the detection sensitivity is required to be reduced. In short, detection of a fault is difficult by deviation monitoring.
Instead of such a method, a model reference method is now drawing attention as disclosed in, for instance, the Japanese Patent Application Publication No. 58279/73. This method uses a model having the input-output characteristics of W(s)=G.sub.1 (s).multidot.G.sub.2(s) /1+G.sub.1(s) .multidot.G.sub.2(s) where G.sub.1(s) is the input-output characteristics of the controls and G.sub.2(s) the input-output characteristics of the plant. This model is impressed with a deviation signal between the model output and the plant output, so that the model output is compared with a feedback signal making up the plant output. W(s) is generally called the overall transfer function. In the model reference method, the model W(s) may take either analog or digital form. If W(s) is complicated, however, it should preferably be carried out digitally by using a micro-computer or like. If the feedback control system which is an object for fault detection has no fault, the signal passed through the model W(s) coincides with the feedback signal and the deviation signal between them is zero no matter what change the target signal undergoes. As compared with the above-mentioned deviation reference method in which the deviation signal changes at the transient time under normal conditions, the model reference method, for lack of any change under normal conditions, has an improved detection sensitivity and has less cases of judgement error.
In order to achieve the effect of the model reference method sufficiently, however, the model characteristics are required to coincide sufficiently with the input-output characteristics of the object for fault detection. For this purpose, model characteristics are designed very carefully to determine model parameters. Nevertheless, it is impossible to attain complete coincidence of characteristics, and also an error is likely to occur more often with the lapse of time due to deterioration of the object for fault detection and the model. In order to obviate this shortcoming, the model is readjusted periodically or seasonally. In view of the fact that the readjustment is complicated and requires much labor and time, however, the model itself should preferably have the function to adjust the model parameters.
In the model reference adaptive system (M.R.A.S.) used for control of airplanes, for example, a method for adjusting model parameters is used in which the model parameters are continuously corrected in such a manner as to minimize the square integration value of the deviation signal between the model output and the object output. In such a system, however, adjustment requires a long time in the case of an object having a large time constant or where many unknown parameters are involved. Also, since the characteristics of the object for detection do not substantially change with time under normal conditions, continuous adjustment as in M.R.A.S. is not required.
The input and output characteristics of the plant have static and dynamic characteristics which must be distinguished from each other in proper model correction. For independent correction thereof, however, information suitable for correcting the characteristics is required, and also it is necessary to determine whether the plant is under operating conditions suitable for collecting such information.